The present invention is related to heat sensitive coating compositions in general, and in particular, to an opaque coating composition whose opacity decreases irreversibly when exposed to a source of heat such as a laser beam, and a related method of marking substrates with a laser beam.
High speed laser beam marking or coding of commercial products, for example, metal cans and plastic products, is a growing area of great interest and offers certain advantages over conventional marking technologies which are generally afflicted with one or more drawbacks. For example, marking by ink jet printing requires frequent maintenance to keep the nozzle from clogging. Further, the use of fluids such as ink jet inks containing solvents in contact with the printed surface cannot be tolerated in certain critical applications for reasons related to safety and compatibility.
In view of the foregoing, laser beam marking systems have received a significant attention from the industry. See, for example, European Patent Application 0 739 933 A1, UK Patent Application GB 2291719 A, and U.S. Pat. Nos. 5,760,120 and 4,861,620. Laser beam marking has the advantage that a fluid is not employed in the marking process. The laser beam marking systems can also be operated with minimal maintenance requirements. However, systems known heretofore suffer from certain shortcomings. For example, in some systems, a polymeric molded product containing a laser sensitive pigment is marked by irradiating with a laser beam. The laser beam creates a mark by evaporating or pyrolyzing the polymeric resin, and, as a result, exposing the pigment. See, e.g., European Patent Application 0 739 933 A1 and U.S. Pat. No. 5,760,120. Such a system, however, can leave behind dust or residues as the polymer material is ablated from the surface of the product. Further, in the above method, since the polymer is etched by the laser beam, the surface of the product is uneven, and, therefore, lacks smoothness.
Thus, there exists a need for a laser marking system that does not create or leave behind dust or residue during marking. There further exists a need for a laser marking system that leaves a relatively smooth surface. There further exists a need for a system that offers a broad range of color contrast. There further exists a need for a system that is amenable in a variety of colors. There further exists a need for a laser marking system that can mark at high speeds, for example, at about 300 feet/minute or higher.
These and other objects of the present invention will be apparent from the detailed description of the preferred embodiments of the invention set forth below.
The foregoing needs have been fulfilled to a great extent by the present invention which provides a heat responsive colorant particle comprising a colorant and a polymeric material whose opacity changes substantially irreversibly when exposed to heat. The present invention further provides a heat markable coating composition comprising a colorant and a polymeric material whose opacity changes substantially irreversibly when exposed to heat. A laser beam can be used to provide the heat. Preferably, the opacity of the polymeric material decreases as a result of exposure to heat and the colorant becomes more visible.
The present invention further provides a method for marking a substrate with a laser beam, the method comprising applying to the substrate the heat markable coating composition to provide a coated substrate and irradiating selected areas of the coated substrate with a laser beam. The present invention further provides a method for preparing a coated substrate suitable for laser marking comprising:
(a) providing a substrate;
(b) coating the substrate with a composition comprising a colorant, a first binder resin, and a first carrier to provide a first coated substrate; and
(c) coating the first coated substrate with a composition comprising a polymeric material whose opacity changes substantially irreversibly when exposed to heat, a second binder resin, and a second carrier to obtain the coated substrate.
While the invention has been described and disclosed below in connection with certain preferred embodiments and procedures, it is not intended to limit the invention to those specific embodiments. Rather it is intended to cover all such alternative embodiments and modifications as fall within the spirit and scope of the invention.